1. Field of the Invention
The present invention relates in general to a passenger restraining device of a motor vehicle, and more particular to an air bag device mounted on a steering wheel for protecting a driver.
2. Description of the Prior Art
Hitherto, various air bag devices have been proposed and put into practical use. Some are of a type for a driver, that is mounted on a boss potion of a steering wheel. Upon a vehicle collision, gas is rushed into the air bag to instantly expand the same thereby to safely hold the upper half of the driver. Some of the air bags are of a type that comprises front and rear circular woven fabrics of Nylon (trade name) whose peripheries are sewed together. Thus, when applied with no gas, the air bag is shaped flat. The front fabric that faces the boss portion of the steering wheel is formed with a gas inlet opening, and a peripheral portion of the gas inlet opening is fixed to the boss portion of the steering wheel. An inflater is also mounted on the boss portion, which has a gas outlet connected to the gas inlet opening of the air bag. In not use, the air bag is kept folded small and housed in a container mounted on the boss portion. Upon a vehicle collision, a certain amount of gas is instantly ejected from the inflater to expand the air bag. During the expansion, the air bag breaks the container and gets out of the same and safely holds the upper half of the driver.
For much safely holding the driver even if he or she has been seated out of position with his or her upper half positioned very close to the steering wheel, Japanese Patent First Provisional Publication 7-149199 proposes a measure that forces the air bag to expand laterally flat. In this disclosed measure, a so-called gas guide fabric is installed in the air bag, by which the gas is guided to flow laterally outward. More specifically, within the air bag, there is installed a circular inner fabric as the gas guide fabric, that is arranged to cover over the gas inlet opening and has a peripheral portion sewed on spaced portions of the front circular woven fabric of the air bag. Upon gas generation, the gas is forced to flow through the sewed spaced portions. For discharging the gas from the air bag, vent openings are provided in the front circular woven fabric around the periphery of the circular inner fabric.
However, even the air bag device of the above-mentioned publication has failed to give users satisfaction. That is, due to its inherent construction, a larger amount of gas is discharged through the vent openings without contributing to expansion of the air bag. This brings about a need of increasing capacity and size of the inflater and lowering the air bag expansion speed. If, for solving this drawback, the vent openings are positioned away from the gas inlet opening, that is, near the periphery of the front circular woven fabric, a new drawback tends to occur. That is, in this case, at a final stage of the air bag expansion, the hot gas discharged from the vent openings may be directed toward the upper half of the driver. Thus, to cool the gas, additional measure becomes needed. Furthermore, due to the increased distance from the gas inlet opening to the vent openings, it becomes necessary to devise a new method of compactly folding the air bag for making it possible for the vent opening to serve properly as an inner pressure controlling means. Furthermore, if the circular inner fabric has a larger size, the rearward swelling of the same caused by rush of the gas through the gas inlet opening becomes remarkable, which affects the expansion characteristics of the air bag, and hence the pressure to be applied to the vehicle passenger.
It is therefore an object of the present invention to provide an air bag device which is free of the above-mentioned drawbacks.
According to a first aspect of the present invention, there is provided an air bag device. The air bag device comprises first and second fabrics having peripheral portions sewed together to constitute a bag structure; a gas inlet opening formed in the first fabric; a third fabric smaller in size than the first and second fabrics, the third fabric being installed in the bag structure in a manner to cover over the gas inlet opening; a plurality of sewed points by which the first and third fabrics are sewed together; a plurality of gas flow passages provided between the first and lo third fabrics, each gas flow passage being defined between neighboring two of the sewed points; and a vent opening that extends through at least one of the sewed points to provide a communication between a major portion of the interior of the bag structure and the outside of the same.
According to a second aspect of the present invention, there is provided an air bag device for use on a steering wheel having a boss portion. The air bag device comprises a holding unit connected to the boss portion of the steering wheel; an inflater held by the holding unit, the inflater including a gas ejection portion; first and second fabrics having peripheral portions sewed together to constitute a bag structure; a gas inlet opening formed in the first fabric, the gas inlet opening having the gas ejection portion of the inflater passed therethrough, so that the gas ejection portion is exposed to the interior of the bag structure; a third fabric smaller in size than the first and second fabrics, the third fabric being installed in the bag structure in a manner to cover over the gas ejection portion of the inflater; a plurality of sewed points by which the first and third fabrics are sewed together; a plurality of gas flow passages provided between the first and third fabrics, each gas flow passage being defined between neighboring two of the sewed points; and a vent opening that extends through at least one of the sewed points to provide a communication between a major portion of the interior of the bag structure and the outside of the same.